1. Field Relating to the Invention
The invention relates to an arrangement for the transmission of information signals by means of pulse code modulation said arrangement comprising a transmitter and a receiver each provided with a decoder. The transmitter is also provided with a quantizing device to which a pulse generator is connected and which supplies output pulses for transmission to the receiver. These output pulses are also applied in the transmitter through a feedback circuit to a transmitter input circuit comprising a cascade arrangement of said decoder and a comparison circuit, said information signals also being applied to this comparison circuit. The transmitter input circuit generates an output signal which is applied to said quantizing device. In this transmission arrangement both the transmitter and the receiver are provided with a dynamic control device to which output pulses from the quantizing device are applied and which supplies a dynamic control signal at its output. The transmitter and the receiver futhermore each comprise a modulator whose output is coupled to an input of the relevant decoder. Furthermore, this modulator has an input which is connected to the output of the relevant dynamic control device, and a controlled first current source circuit whose control input is coupled to said input of the modulator and which supplies an output current as a supply current to a difference amplifier having a first and a second output circuit, which difference amplifier is controlled by output pulses from the quantizing device. The said first and second output circuits of the difference amplifier are connected to an input of current-controlled second and third current source circuits, the output of the second current source circuit being connected directly and the output of the third current source being connected through a precision polarity reversing circuit, to the output of the modulator.
In this connection pulse code modulation is not only understood to mean the transmission of information signals by means of multi-bit code words in the manner as is effected in PCM-coding and transmission, but is also understood to mean the differential coding forms DPCM, delta modulation and delta sigma modulation.
2. Description of the Prior Art
An arrangement of the kind mentioned above is described in U.S. Pat. No. 3,868,574. As described in the said patent application the modulator supplies a positive and a negataive output current. These output currents are utilized for decoding the output pulses supplied by the quantizing device and this modulator supplies a positive output current if the quantizing device supplies an outputpulse having for example the logical value 1 and it supplies a negative output current if the quantizing device supplies an output pulse having the complimentary logical value, that is to say, the logical value 0. The absolute value of an output current of the modulators is dependent on the value of its input current, or in other words it is dependent on the value of the output current of the associated dynamic control circuit.
In the arrangement described in said U.S. Patent the current-controlled second and third current source circuits as well as the precision polarity reversing circuit are formed as so-called current mirror circuits. In connection with the requirement to be able to form the modulators as an integrated circuit (monolithic) the first current source circuit and the difference amplifier are more particularly formed with npn-transistors and consequently the current-controlled second and third current source circuits are each formed with pnp-transistors and the precision polarity reversing circuit is formed with npn-transistors.
when forming the modulator as a monolithic integrated circuit with a polarity reversing circuit built up of npn-transistors, it is achieved that the positive output current occurring at a given value of the input current of the circuit is accurately equal to the negative output current occurring at the same value of the input current.
In monolithic integrated circuits, however, the combination of pnp and npn-transistors causes troubles residing in the fact that the pnp-transistors generally have a too low current gain factor (for example less than 10) while this current gain factor in pnp-transistors is also temperature and current-dependent and may be different from integrated circuit to integrated circuit.
Owing to these properties of the pnp-transistors incorporated in the integrated circuits, the relationship between input current and output current may vary greatly from integrated circuit to integrated circuit. As a result serious deviations of the desired linearity of the signal transmission occur which cannot be inhibited to a sufficient extent with the aid of the control members present in the transmission device.